**Core Concept**
Mycobacterium tuberculosis (M. tuberculosis) is a Gram-positive, acid-fast bacillus that causes tuberculosis (TB), a serious infectious disease affecting the lungs and other parts of the body. The bacterium is characterized by its unique cell wall, which contains mycolic acids and other lipids that make it resistant to decolorization with acid-alcohol.

**Why the Correct Answer is Right**
M. tuberculosis is an obligate aerobe, meaning it requires oxygen to grow and multiply. The bacterium uses the oxygen-dependent respiratory chain to generate energy, which is essential for its survival and pathogenesis. The enzyme responsible for the conversion of oxygen to water is cytochrome c oxidase, which is also the target of the antibiotic isoniazid.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because M. tuberculosis is not an anaerobe; it requires oxygen to grow and multiply.
**Option B:** This option is incorrect because M. tuberculosis is not a facultative anaerobe; it cannot switch between aerobic and anaerobic metabolism.
**Option C:** This option is incorrect because M. tuberculosis is not a strict anaerobe; it requires oxygen to grow and multiply.

**Clinical Pearl / High-Yield Fact**
M. tuberculosis is a highly contagious pathogen that can be transmitted through the air when an infected person coughs or sneezes, releasing droplets that contain the bacteria. The risk of transmission can be minimized by using masks, maintaining good ventilation, and administering effective treatment to patients with active TB.

**Correct Answer: D. M. tuberculosis is an obligate aerobe.**